Electroacoustic transducer



May 17, 1966 E. v. CARLSON ELECTROACOUSTIG TRANSDUCER 2 Sheets-Sheet 1 Filed 001'.. 27, 1961 May 17, 1966 E. v. CARLSON ELEGTROACOUSTIC TRANSDUCER 2 Sheets-Shea?l 2 Filed OCT.. 2'?, 1961 3 4 5 e 7691 INVENTOR.

fzef? @wdm FREQUENCY /N CYCLES P5K 5560A@ This invention 'relates to electroacoust-ic transducers and yparticularly to a miniature microphone 'whose diaphragm operates at a high natural .resonance and to means for improving the response in the sub-resonant frequencies.

In microphones of 'the 'magnetic or dynamic 'type where the electrical output voltage or current i's roughly proportional Ito the mechanical velocity in the transducer drive United States Patent O element, which in turn responds to the pressure of a Y Asound wave striking a diaphragm, the electrical response at sufficiently low frequencies is 'approximately proporti-onal to frequency. This is lthe `res-ult of ta stiffness being the prinoip'al restraint 'controllingythe motion at these 'low frequencies. In large or moderate-sized microphones, ,this controlling stiflness resides principally in the mechanical elements of the construction. Where the size of the housing is not critica-l, 'as in large microphones, the back cavity may be such that with proper mass and compliance of -the diaphragm, the natural resonance of the diaphragm may be in the region 'of yZOOcycles per second. Sounds below 'this frequency will generate signals .roughly proportional to frequency.

As lthe Irnicrophone is made smaller, the black cavity becomes smaller |and with 'the decline in 'diaphragm size, the natural resonance of the diaphragm rises with the result that the decline in response commences ata higher frequency. In a Icase 'lifX 1/2 X 1% the natural resonance of the `diaphragm may be about 1700 cycles, and the decline in response .as the frequency declines is almost.

linear. Since the range of principal frequencies of the human voice is 'from about 80 cycles to i100 cycles, and since the fundamental frequency 'of the speaking voice lof most people is from albout the middle of the base range to about the m-iddle of the soprano range, namely, from about 180-6()0 cycles per second, it is evident that the current generated in the microphone motor is in the range of low eiiiciency and is not uniform for the various voices. The response for Isuch la microphone is indicated by a line A in FIGURE 9.

To improve the response in the lower frequencies, 'Thuras taught that `the output voltage for a constant acoustic sound wave pressure, that is, the sensitivity, could be enhanced fb-y the addition of lan inertance tube communicating the exterior of the microphone with the cavity enclosed by the diaphragm or communicating between sections of the cavity divided by Ia diaphragm. In the accepted practice, the inertance tube connects the outside spiace, or a chamber communicating with the outside space, to `a cavity enclosed by the diaphragm. Under the influence of sound, two effects 'are provided, namely, (l) airis allowed to pas-s from one side of lthe ydiaphragm to the other, land (2) the configuration is such that 1an appreciable portion of the energy is stored in the inertance of this air. The'volum'e and lengt-h of the vtube lare chosen to lprovide anatural resonance with the associated compliance below *the natural resonance of the diaphragm.

yA response tylpical of the effect of an inert'ance tube in .a miniature transducer is illustrated by the curved line B inFIGURE 7. However, when the case is reduced to perhaps V16" X Mt x 7/16, 'a major portion of its volume v is occupied by the transducer motor landthere is no room flor the linertance :tube which must have some compromise between decreased cross section or 'increased length. An inert'ance tube becomes more efficient 'as its cross-sectional Patented May 17, 1,966

area and length increase. As its bore is decreased, the frictional effects of the .air against the walls of the inertance tube place a limit on the performance of such a tube, that is, a disproportionate lamount of energy is dissipated as that compared to that stored in the inertia of the air.

The principal object Vof this invention is to provide 'a `means 'of :achieving benefits of the teachings of Thuras in a microphone too small to provide spacefor an'inertance tube 'of the requisite size.

y In this invention, these effects are lachieved by providing the Adiaphragm with an inert'ance diaphragm portion land a signal translating or acting diaphragm portion so that, i'or example, an excess pressure on the outside of this arrangement causes the. inerta-nce diaphragm. portion 'to deflect inwardly [and in its ste-ad accept a quantity of air.

,The opposing surface of this diaphragm portion then displaces an equivalent 4amount of air, which must be absorbed in the remainder lof the cavity, as muchas if it hadentered through a port. By securing ya mass to the inertance 'diaphragm portion, the motion of this portion Awill accelerate the mass and store kinetic energy, vrel-ated to the rate 'at which air is displaced fby the inertance vdiaphragm portion in much the same manner that the inertia of the air in the inertance .tube stores energy. As there are no narrow passages to absorb energy from the ymovementof |air lby friction and the spring-like action 'of the inertance diaphragm portion converts very little energy to heat, by this construction, it is practicalto overcome the ydisadvantages of the acoustic inertance tube.

Thus it is seen that the inertance diaphragm portion and mass can be constructed as ra segment ofthe diaphragm acting upon the transducer.

' In an inertance tube of suiiiciently large bore to render the frictional or damping forces unimportant, the acoustic inerta-nceis given by the relation mass of 'air in tube Inertance=, (cross section area of tube) 2 Similarly theinertan'ce of the structure in this invention is combined mass of the inertance diaphragm portion and loading weight (eifective area of inertance diaphragm portion) 2 Inertance:

-the sensitivity is observed. At the frequencies immediately lower than the resonant frequency of the weighted inertance diaphragm portion, the motion of this portion is such that the increment-al pressure generated within the closed chamber below the inertance diaphragm portion is caused -to approach the pressure outside the chamber with a resultant decrease-in sensitivity. Hence, the in- Vertancediaphragm portion is weighted to provide-a resonance of 500 cycles or less. The enhanced response lis typically shown by the line C in FIGURE 7.

v:In attaining this general object a specific objecthas been in mind. Since the over-all volume of 4the case of the microphone is only slightly larger than the motor, diaphragm and drive yconnection between the two, the configuration of the case will conform very closely to Athe over-all configuration of these major three components. Where the motor design provides one pair of dimensions which exceed the area required for the acting or signal 3 diaphragm portion, a specific object is to have the inertance diaphragm portion lie in substantially the same plane. Embodiments I and II illustrate structures that achieve this object.

Another object is to damp out the response at very low :frequencies or below about 200 cycles in a microphone intended prirna-rily for hearing aid use.

These and such other objects as may hereinafter appear are attained by the embodiments of the invention hereinafter described in connection with the following drawings:

FIGURE 1 shows a perspective view of Embodirnent I of the invention, with the upper portion of the case-and I the lid removed;

FIGURE 2 shows a longitudinal section of a transducer with the diagram of FIGURE 1 for the purpose of illust-rating schematically the relative sizes of the motor and case;

FIGURE 3 is a plan View of the diaphragm of FIG- URE 1;

FIGURE 4 is a section similar to the upper portion of FIGURE 2, but showing a moditicationl of the diaphragm of Embodiment I;

FIGUR-ES 5 and 6 are perspective and longitudinal sectional views of Embodirnent II; and,

FIGURE 7 is a graph illustrating the enhancement of the response due to the inertance diaphragm portion at certain frequencies.

Embodiment I Referring to FIGURES l through 3 of the drawings, 11 is an open-sided cup which constitutes the principal portion of a transducer case. The open side of this cup is en'- closed by a bulkhead 12 which -acoustically is unimportant because of openings such as a drive pin opening 14. In some transducers, there is no bulkhead 12, the motor being iixed to the cup 10 by other means. In the structure shown, however, the motor 16 is mounted on t-he bulkhead 12 by screws 18 and 20.

The dimensions of the motor 16 are defined schematically by end Walls 22 and 24, bottom 26, and side walls 28 and 30, see FIGURES 2 and 3. These walls do not indicate physical parts of the motor. Rather they indicate the over-all limits of the motor, and as shown they do not permit the introduction of an inertance tube of such diameter as to have a resonant frequency of 400 cycles per second or less.

Mounted on t-he perimeter of the bulkhead l12, or if there is no bulkhead, upon the perimeter of the opening of the cup 10, isa diaphragm 32. Adjacent the mounting is a perimetric Wale 34, formed of thin material. This Wale 34 provides flexibility for the diaphragm, 4i.e., the compliance of the diaphragm, so that it may vibrate in response to sound, as in -a microphone, or in response to a drive pin, as in a receiver. The diaphragm is transversely divided by a Wale 36 to provide a drive or driven portion 38, called the acting diaphragm portion, and a portion 40, called the inertance diaphragm portion. The etfect of the Wale 36 lis to insulate movements of the portion 38 from the movements of the portion 40, that is, assuming no variable air pressure applied to the inertance diaphragm portion, vibration of the acting diaphragm portion will communicate substantially no lmovement to the inertance diaphragm portion.

Mounted on the acting diaphragm portion 38 is a plate 42 which is a piston tending to restrict movement of the main portion of the acting diaphragm portion 38 to a direction normal to its top surface and to limit flexing or compliance to the wales 34 and 36. The acting diaphragm portion 38 is connected to the motor 16 by a drive pin 39.

The resonance of the acting diaphragm portion 38 is in .the neighborhood of 1500 cycles or hi-gher. This is due to the motor used, the weights of the moving elements and to the small air volume of the back cavity 44. The back cavity is not quite tight as it is necessary to permit pas- L4, sage of air slowly to and from the back cavity to equalize the pressure when atmospheric pressures change.

Formed in that portion of the diaphrag-m 32 `between the Wale 36 and the balance of Wale 34 is a platform or table 46 which constitutes the major portion of the inertance diaphragm portion 40. The line of joinder of the perimetric 4wall 48 of this table 46 assists in insulating it from any vibration transmitted to the Wale 36 by the acting diaphragm por-tion 38. Mounted on the table 46 is a weight 449. This Weight 49 is selected to provide a natural resonant lfrequency of the inertance diaphragm portion 40 of typically 400 cycles per second or less. Tungsten and tungsten alloys have been found to be de sirable materials lfor this weight because of their densities. To attain -this inertance, one follows the formula Inertance:

combined mass of diaphragm and Weight (etfective area of `the diaphragm)2 With .an inertance diaphragm potrion having a natural resonance of 400 cycles per second, the enhancement of 'the response of the acting diaphragm portion 38 having a natu-ral resonance of perhaps 1500 cycles per second is illust-rated in FIGURE 7 by the curved line C.

The acting diaphragm portion 38 and the table 4-6 of lthe inertance diaphragm portion 40 lie substantially in a common plane and hence substantially no :additional volume is required in the case. The use of a flat Weight 49 assists in conserving space.

Mounted over the diaphragm 32 is a cover 50 having a sound opening 52.

A modification of the first embodiment of the invention is shown in FIGURE 4. This structure differs from that in FIGURES l through 3 only in the rib 58 mounted on the bulkhead 12 and the diaphragm 60 rigidly mounted on the rib 58. This diaphragm has two transverse wales 61 and 62 and the inertance diaphragm portion 63 vis more decisively rendered independent of the acting diaphragm portion andthe alignment of the pin with its point through 3 or that of FIGURE 4 is used depends in part upon the geometry of the particular motor and case used. The drive pin 39 should be connected to the acting diaphragm portion and the alignment of the pin with its point of connection to, for example, a point on an armature, will vary in accordance with the position of other components of the motor.

Embodiment Il The second embodiment is shown in FIGURES 5 and 6, tand is a modification of the rst embodiment. Here. the diaphragm 82 is supported by -a perimetric exible wale 84 which is reinforced by a piston member 86. The diaphragm 82 has an acting diaphragm portion Which constitutes the major portion thereof and will be described hereinafter. Formed entirely within the Wale 84 is an inertance diaphragm 90 consisting principally of =a table or platform 92 upon which is disposed a flat Weight 94, selected in accordance with previously described procedures. This diaphragm vibrates as a whole but weight 94 moves in accordance with the resonant frequency of itself and its table to produce much the same result as obtained in Embodirnent I.

Referring to FIGURE 7, it will be seen that at very low frequencies, curve C approaches curve A .and is substantially above curve B. The reduced sensitivity at very low frequencies shown in curve B arises because of the low impedance to air flow through the inertance tube at these frequencies. These very low frequencies are objectionable in many speech-reproducing systems and specically in hearing aids Where they may arise from mechanical vibration of the microphone, due to body and clothing movements, as well as to sources of low frequency sound which make negligible contribution to speech intelligibility, and are regarded as vobjectionable noises by the user. there is provided in the acting diaphragm portion of diaphragm 82 a tube or passage with a narrow cross section to interconnect the front and rear surfaces of the diaphragm. Two such tubes or passages 96 and 98 are shown in FIGURES 5 and 6. TWO are used to provide dynamic balance to the diaphragm. Because of the importance of conserving space, these passages are preferably mounted in the diaphragm as shown here and in United States Letters Patent No. 2,966,558, dated December 27, 1960, to Hugh S. Knowles. To achieve the result desired here, the tube or passage cross section is made substantially thinner than i-t would be to function :as a conventional inertance tube. Its impedance is predominantly resistive and made high enough not to signicantf ly reduce the sensitivity of curve C, in the frequency region Where it lies above curve A, and yet such that sensitivity at very low frequenciesis substantially reduced.

The usefulness of the invention is in transducers Wherein the case does not permit use of an inertance tube and wherein the acting diaphragm has a natural resonance of above 1000. In recent years, the natural resonance of the acting diaphragm on transducers Worked on by applicant has been closer to 1500 cycles and in the extremely small ones, the natural resonance may be as high as 2500 cycles. The inertance diaphragm is Weighted to produce a resonant frequency of less than 700 cycles and a resonant frequency of 300 to 400 cycles has produced excellent results.

In the claims, the Word Wale means a compliant rim of a diaphragm. In speakers, it is sometimes referred to as a sciver or a surround I-ts cross section may be varied. The Word housing refers to the case 11 which may be inside another case. Significantly, housing defines the Walls of the back or motor cavity. Inertance diaphragm identifies one that vibrates freely Within its own characteristics.

Having thus described his invention, applicant claims:

To attenuate these very W frequencies,

1. An electroacoustic transducer comprising a housing having an open side; a motor mounted in said housing; a diaphragm closing said open side of said housing, said diaphragm having an acting diaphragm portion and a 5 weighted inertance diaphragm portion; and drive means connecting said acting diaphragm portion to said motor.

2. The electroacoustic transducer of claim 1 wherein said diaphragm is provided with a Wale between said acting diaphragm portion and said inertance diaphragm por- 10 tion to insulate movements of said acting diaphragm portion from movements of said inertance diaphragm portion.

3. The electroacoustic transducer of claim 1 wherein said housing is provided with a rib for engaging and supporting said diaphragm along a line separating said acting diaphragm portion and said inertance diaphragm portion.

4. The electroacoustic transducer of claim 1 wherein the natural resonance of the acting diaphragm portion is Within the range of 1000 to 2500 cycles per second and the natural resonance of the inertance diaphragm portion is less than 700 cycles per second.

5. The electroacoustic transducer of claim 4 wherein the natural resonance of the inertance'diaphragm portion is Within the range of 300 to 400 cycles per second.

References Cited by the Examiner UNITED STATES PATENTS 1,988,250 1/1935 Olson 179-116 2,327,136 8/1943 Shapiro 179-107 3 2,627,558 2/1953 Wiggins 179-1155 2,713,396 7/1955 Tavares 181-31 2,848,561 8/1958 Gorike 179-180 2,966,558 12/1960 Knowies 179-114 ROBERT H..ROsE, Primary Examiner. WILLIAM C. COOPER, Examiner.

R. M. GOLDMAN, F, N. CARTEN, Assistant Examiners. 

1. AN ELECTROACOUSTIC TRANSDUCER COMPRISING A HOUSING HAVING AN OPEN SIDE; A MOTOR MOUNTED IN SAID HOUSING; A DIAPHRAGM CLOSING SAID OPEN SIDE OF SAID HOUSING, SAID DIAPHRAGM HAVING AN ACTING DIAPHRAGM PORTION AND A WEIGHTED INERTANCE DIAPHRAGM PORTION; AND DRIVE MEANS CONNECTING SAID ACTING DIAPHRAGM PORTION TO SAID MOTOR. 